Projection objective of a microlithographic projection exposure apparatus

ABSTRACT

A projection objective of a microlithographic projection exposure apparatus comprises a manipulator for reducing rotationally asymmetric image errors. The manipulator in turn contains a lens, an optical element and an interspace formed between the lens and the optical element, which can be filled with a liquid. At least one actuator acting exclusively on the lens is furthermore provided, which can generate a rotationally asymmetric deformation of the lens.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 11/971,328, filed Jan. 9, 2008, now U.S. Pat. No. 7,830,611, whichclaims priority to PCT International Application No. PCT/EP2006/007273,filed on Jul. 24, 2006, which claims priority to U.S. ProvisionalApplication Ser. No. 60/702,137, filed Jul. 25, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to projection objectives of microlithographicprojection exposure apparatus. Such apparatus are used for theproduction of large-scale integrated circuits and other microstructuredcomponents. The invention relates in particular to projection objectivescomprising a manipulator for reducing rotationally asymmetric imageerrors.

2. Description of Related Art

Microlithography (also called photolithography or simply lithography) isa technology for the fabrication of integrated circuits, liquid crystaldisplays and other microstructured devices. The process ofmicrolithography, in conjunction with the process of etching, is used topattern features in thin film stacks that have been formed on asubstrate, for example a silicon wafer. At each layer of thefabrication, the wafer is first coated with a photoresist which is amaterial that is sensitive to radiation, such as deep ultraviolet (DUV)light. Next, the wafer with the photoresist on top is exposed toprojection light through a mask in a projection exposure apparatus. Themask contains a circuit pattern to be projected onto the photoresist.After exposure the photoresist is developed to produce an imagecorresponding to the circuit pattern contained in the mask. Then an etchprocess transfers the circuit pattern into the thin film stacks on thewafer. Finally, the photoresist is removed. Repetition of this processwith different masks results in a multi-layered microstructuredcomponent.

A projection exposure apparatus typically includes an illuminationsystem, a mask alignment stage for a aligning the mask, a projectionlens and a wafer alignment stage for aligning the wafer coated with thephotoresist. The illumination system illuminates a field on the maskthat may have the shape of an rectangular slit or a narrow ring segment.

In current projection exposure apparatus a distinction can be madebetween two different types of apparatus. In one type each targetportion on the wafer is irradiated by exposing the entire mask patternonto the target portion in one go; such an apparatus is commonlyreferred to as a wafer stepper. In the other type of apparatus, which iscommonly referred to as a step-and-scan apparatus or scanner, eachtarget portion is irradiated by progressively scanning the mask patternunder the projection light beam in a given reference direction whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection. The ratio of the velocity of the wafer and the velocity ofthe mask is equal to the magnification of the projection lens, which isusually smaller than 1, for example 1:4.

It is to be understood that the term “mask” (or reticle) is to beinterpreted broadly as a patterning means. Commonly used masks containtransmissive or reflective patterns and may be of the binary,alternating phase-shift, attenuated phase-shift or various hybrid masktype, for example. However, there are also active masks, e.g. masksrealized as a programmable mirror array. An example of such a device isa matrix-addressable surface having a viscoelastic control layer and areflective surface. More information on such mirror arrays can begleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No.5,523,193. Also programmable LCD arrays may be used as active masks, asis described in U.S. Pat. No. 5,229,872. For the sake of simplicity, therest of this text may specifically relate to apparatus comprising a maskand a mask stage; however, the general principles discussed in suchapparatus should be seen in the broader context of the patterning meansas hereabove set forth.

One of the essential aims in the development of projection exposureapparatus is to be able to lithographically generate structures withsmaller and smaller dimensions on the wafer. Small structures lead tohigh integration densities, which generally has a favorable effect onthe performance of the microstructured components produced with the aidof such apparatus.

The size of the structures which can be generated depends primarily onthe resolution of the projection objective being used. Since theresolution of projection objectives is inversely proportional to thewavelength of the projection light, one way of increasing the resolutionis to use projection light with shorter and shorter wavelengths. Theshortest wavelengths currently used are 248 nm, 193 nm or 157 nm andthus lie in the (deep) ultraviolet spectral range.

Another way of increasing the resolution is based on the idea ofintroducing an immersion liquid with a high refractive index into animmersion interspace, which remains between a last lens on the imageside of the projection objective and the photoresist or anotherphotosensitive layer to be exposed. Projection objectives which aredesigned for immersed operation, and which are therefore also referredto as immersion objectives, can achieve numerical apertures of more than1, for example 1.4 or even higher.

The correction of image errors (i.e. aberrations) is becoming more andmore important for projection objectives with particularly highresolution. Many ways in which image errors can be corrected inprojection objectives are known in the prior art.

The correction of rotationally symmetric image errors is comparativelystraightforward. An image error is referred to as being rotationallysymmetric if the wavefront deformation in the exit pupil is rotationallysymmetric. The term wavefront deformation refers to the deviation of awave from the ideal aberration-free wave. Rotationally symmetric imageerrors can be corrected, for example, at least partially by movingindividual optical elements along the optical axis.

Correction of those image errors which are not rotationally symmetric ismore difficult. Such image errors occur, for example, because lenses andother optical elements heat up rotationally asymmetrically. One imageerror of this type is astigmatism, which may also be encountered for thefield point lying on the optical axis. Causes of rotationally asymmetricimage errors may, for example, be a rotationally asymmetric, inparticular slit-shaped, illumination of the mask, as is typicallyencountered in projection exposure apparatus of the scanner type. Theslit-shaped illumination field causes a non-uniform heating of theoptical elements, and this induces image errors which often have atwofold symmetry.

However, image errors with other symmetries, for example threefold orfivefold, or image errors characterized by completely asymmetricwavefront deformations are frequently observed in projection objectives.Completely asymmetric image errors are often caused by material defectswhich are statistically distributed over the optical elements containedin the projection objective.

In order to correct rotationally asymmetric image errors, U.S. Pat. No.6,338,823 B1 proposes a lens which can be selectively deformed with theaid of a plurality of actuators distributed along the circumference ofthe lens. Since the two optical surfaces of the deformable lens arealways deformed simultaneously, the overall corrective effect isobtained as a superposition of the individual effects caused by the twodeformed optical surfaces. This is disadvantageous because it is therebyvery difficult to correct a particular wavefront deformation, which isdetermined by measurements or simulation, and has been generated by theother elements of the projection objective. The individual effects ofthe two deformed surfaces furthermore partially compensate for eachother, so that the lens has to be deformed quite strongly in order toobtain a sufficient corrective effect.

US Pat. Appl. No. 2001/0008440 A1 discloses a manipulator suitable tocorrect image errors for a projection objective, in which two membranesor thin plane-parallel plates enclose a cavity. The membranes or platescan be deformed by varying the pressure of a fluid (a gas or a liquid)contained in the cavity. A rotationally asymmetric deformation can beachieved by rotationally asymmetric framing of the membranes or plates.A disadvantage with this known manipulator, however, is that only thefluid pressure is available as a variable parameter. This implies, forexample, that the symmetry of the deformation is once and for all fixedby the framing of the membranes or plates and thus cannot be changed.

WO 2006/053751 A2 discloses various adjustable manipulators for reducinga field curvature. In some embodiment optical elements such as lenses ormembranes are deformed by changing the pressure of liquids adjacent theoptical elements.

U.S. Pat. No. 5,665,275 discloses a prism which has a variable prismangle. A variation of the prism angle is achieved using twoplane-parallel plates which can be mutually deflected via a deformableconnecting ring. The interspace defined by the plates and the connectingring is filled with an organic liquid. The connecting ring is deformedby an actuator engaging on it. This device is particularly suited for ananti-vibration optical system in a photographic system.

U.S. Pat. No. 5,684,637 discloses a spectacle lens having a cavity whichis filled with a liquid. For the correction of rotationally asymmetricimage errors, for example astigmatism, deformable membranes are providedthat have a non-circular circumference. Since always a plurality ofoptical surfaces are simultaneously deformed by changing the pressure ofthe liquid, it is difficult to widely correct a certain (measured)wavefront deformation without introducing a plurality of otherdeformations.

There are many other liquid lenses in the prior art having deformablelens surfaces for changing the focal length. With these known liquidlenses the deformation is always rotationally symmetric, and hence theyare not suitable for correcting rotationally asymmetric image errors.Examples of such variable focal length lenses can be found in JapanesePat. Appls. JP 811 4703 A, JP 2002 131 513 A and JP 2001 013 306 A, inEP 0 291 596 B1 and in U.S. Pat. No. 4,289,379.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a projectionobjective of a microlithographic projection exposure apparatus, whichhas a manipulator for reducing rotationally asymmetric image errors.More particularly, the manipulator shall have a simple design and shallmake it possible to correct a large variety of rotationally asymmetricfield or pupil related image errors.

According to the invention, this object is achieved by a projectionobjective having a manipulator, which comprises:

-   -   a) a first optical element of a refractive type,    -   b) a second optical element,    -   c) an interspace formed between the first optical element and        the second optical element,    -   d) a liquid filling the interspace, and    -   e) at least one actuator which is coupled to the first optical        element such that operation of the at least one actuator causes        a rotationally asymmetric deformation of the first optical        element

Since the at least one actuator acts exclusively on the first opticalelement, the at least one actuator deforms both surfaces of the firstoptical element without simultaneously causing an (at least generally)undesired deformation of the second optical element. However, onesurface of the first optical element contacts the liquid, and thus adeformation of this surface has only a very small optical effect if theratio of the refractive index n_(E) of the first optical element and therefractive index n_(L) of the liquid is close to 1. With a refractiveindex ratio n_(E)/n_(L)=1, a deformation of this surface has no opticaleffect at all.

A deformation of the first optical element by the at least one actuatortherefore changes only the optical effect of the other surface of thefirst optical element, i.e. the surface pointing away from the liquid.This is true exactly for a refractive index ratio n_(E)/n_(L)=1, but isat least substantially true with a refractive index ratio n_(E)/n_(L)≈1,for example 0.99<n_(E)/n_(L)<1.01. Even with a larger or smallerrefractive index ratio, for example n_(E)/n_(L)=1.1 or 0.9, there is asubstantial reduction of the optical effect at the interface between theliquid and the optical element if compared to a situation where thedeformed first optical element is surrounded by a gas having arefractive index of 1. For the sake of simplicity and conciseness it istherefore assumed that the refractive index ratio n_(E)/n_(L) equals 1.

Since the manipulator makes it possible to deform effectively only onesingle optical surface, the corrective effect can be adjusted muchbetter. The generally undesired superposition of the optical effectscaused by two simultaneous deformations, as always occurs in the priorart, is avoided.

The use of one or more actuators is a significant advantage incomparison to prior art solutions. This is because only the provision ofactuators fully exploits the potential and design freedom of a singledeformable optical surface. With prior art manipulators, in whichoptical surfaces are deformed only by changing the liquid pressure, itis not possible to produce complicated and varying deformations of thisoptical surface. However, according to the present invention it may beenvisioned to additionally change the pressure of the liquid formodifying the deformation.

By suitably arranging and driving the actuators, it is thus possible tocorrect not only simple rotationally asymmetric wavefront deformations,but even fairly complex wavefront deformations which can bemathematically described as a superposition of higher Zernikepolynomials.

In this context, the term “liquid” is also intended to include highlyviscous gels or the like. The liquid must merely have the property thatit is sufficiently transparent at the wavelength of the projection lightand does not (substantially) transmit a deformation of the first opticalelement onto the second optical element.

The term “deformation” is meant to denote the difference between twoshapes of the first optical element in two different actuation states ofthe one or more actuators. Therefore a rotationally asymmetricdeformation does not necessarily imply that the first optical element isnonrotationally deformed after changing the actuation states. Forexample, the first optical element may have a rotationally asymmetricshape before, and the actuator deforms the first optical element suchthat it receives a rotationally symmetric shape.

The term “rotationally asymmetric” is used herein in the sense of “notaxisymmetric”. Thus a shape is referred to as being rotationallyasymmetric if rotation around a reference axis around an arbitrary angleresults in a different shape. A shape is referred to as having an m-fold(rotational) symmetry if there are only m angles for which a rotationaround the reference axis results in the same shape. A shape having anm-fold rotational symmetry is therefore nevertheless rotationallyasymmetric in this sense.

The first optical element of the manipulator may be designed as aplane-parallel plate. Such a plane-parallel plate is highly suitable asa deformable element because it is then simpler to calculate the forcesthat must act externally on the plate in order to generate a desireddeformation. The thinner such a plane-parallel plate is, the smallerthese forces are. A very thin plate, however, may have the disadvantagethat gravity disadvantageously bends the plate, and a very thin platemay possibly even react too sensitively to the application externalforces. Plate thicknesses in the range of a few millimeters havetherefore been found to be particularly favorable, although this in noway precludes the use of thinner or thicker plates or of thin membranes(pellicles) as deformable first optical element.

The interspace which can be filled with a liquid may have, in adirection along an optical axis of the projection objective, a maximumthickness of less than 1 mm, and preferably less than 2 μm. A smallamount of liquid has the advantage that pressure or temperaturevariations in the liquid affect the optical properties of themanipulafor less strongly.

Depending on the number, arrangement and design of the actuators actingon the first optical element, any deformations of the first opticalelement can be achieved within wide limits. In order to correct imageerrors, however, it is usually sufficient to generate deformationshaving an m-fold symmetry or a superposition of a plurality of m-foldsymmetries, where m=2, 3, 4, . . . .

In principle, the at least one actuator may engage on the first opticalelement in the ways known in the prior art. If the at least one actuatorexerts tensile or compressive forces on the first optical element in theradial direction, as is described in EP 0 678 768 A, then the thicknessof the first optical element will be changed rotationally asymmetricallywithout substantial bending being caused. Often a bending of the firstoptical element is more desirable, which implies that the actuators areconfigured to exert bending moments on the first optical element. Theapplication of forces in a direction other than the radial direction isdescribed in more detail in U.S. Pat. No. 6,388,823 B1.

For rotationally asymmetric image errors which are caused by aslit-shaped illumination field, the symmetry of the image errors andtherefore the symmetry of the deformations required for the correctioncan be predicted very well. The actuators may then be arranged along acircumference of the first optical element and distributed at theappropriate positions. If the manipulator is also meant to correct otherimage errors, the symmetry of which is not known beforehand, then it isexpedient to configure the actuators such that at least two deformationsof the first optical element can be generated with different symmetry.

The second optical element may be of a reflective type (i.e. a mirror).In general, however, the second optical element will be a furtherrefractive optical element which may be curved on one or both sides soas to form a lens, or it may also be designed as a plane-parallel plate.Actuators may also engage on the second optical element in order togenerate an additional deformation. This deformation may be rotationallysymmetric or asymmetric.

In order to achieve optimal decoupling of the first optical element fromthe second optical element, the manipulator may have a pressureequalizer for maintaining a constant pressure of the liquid in theimmersion space. In the event of a deformation of the first opticalelement, this prevents compressive forces from being transmitted throughthe liquid onto the second optical element, where they may cause anundesired deformation.

In the simplest case, the pressure equalizer is a pressure equalizingcontainer which communicates with the interspace through a sealedchannel. A liquid level is formed against a surrounding gas volume inthe pressure equalizing container. If the volume of the interspace isincreased in the event of a deformation of the first optical element,then liquid can flow in from the equalizing container. In the event of avolume reduction, liquid flows back through the channel into theequalizing container. The pressure in the interspace is then equal tothe surrounding gas pressure plus the hydrostatic pressure of the liquidin the equalizing container.

If the manipulator is arranged in or in close vicinity of a pupil planeof the projection objective, the same corrective effect can be achievedfor all field points. The manipulator lies in the vicinity of a pupilplane when the deformable surface of the first optical element has avertex lying at a distance from a pupil plane such thath_(cr)/h_(mr)<0.5. Here, h_(cr) is the height of a central ray whichpasses through the object plane at a maximal distance from the opticalaxis, on the one hand, and through the middle of the pupil plane, on theother hand. The value h_(mr) is defined as the height of a marginal raywhich passes through the object plane on the optical axis, on the onehand, and through the pupil plane at its margin, on the other hand. Thedeformable surface of the first optical element is referred to of beingarranged in the immediate vicinity of a pupil plane whenh_(cr)/h_(mr)<0.15.

However, the manipulator may equally be arranged at other positionswithin the projection objective. For correcting field dependent imageerrors such as field curvature, the manipulator should be arranged in orclose to a field plane, for example the object or image plane of theprojection objective. If the projection objective has an intermediateimage plane, this would be an ideal position for a manipulatorcorrecting field dependent image errors.

In an advantageous embodiment the actuator is fastened to a front or arear surface of the first optical element. The actuator is configured toproduce on the first optical element compressive and/or tensile forcesalong directions that are at least substantially tangential to thesurface. Such an actuator is advantageous because it does not require anadditional rigid body on which the actuator rests.

Such an actuator may be realized as a layer which is supplied to thesurface. The layer has a variable dimension along at least one directiontangential to the surface. Layers having such a property may be formed,for example, by piezo electric materials, or by materials having acoefficient of thermal expansion that is different from the coefficientof thermal expansion of the first optical element.

In the latter case the temperature of the material may be controllablychanged with the help of a device that may be configured to directradiation onto the layer, or to apply a voltage to the layer, forexample.

A wide variety of deformations may be produced with a plurality of layeractuators having the shape of ring segments. The ring segments form acircular ring which is centered with respect to an optical axis of theprojection objective.

Thin actuator layers furthermore are particularly suitable to distributethe actuators over an optical surface within an area through whichprojection light propagates. If such actuators are opaque at thewavelength of the projection light, the manipulator has to be arrangedin or in close proximity to a pupil plane of the projection objective.If the actuators are at least substantially transparent at thewavelength of the projection light, also positions outside a pupil planemay be contemplated.

It is to be understood that the aforementioned (layer) actuatorproducing compressive and/or tensile forces may advantageously be usedalso in other kind of optical manipulators, for example manipulatorswhich do not contain liquids and/or which produce only symmetricdeformations.

According to another embodiment of the invention, one or more additionalsensors are fastened to a front or rear surface of the first opticalelement. The sensors are configured to measure a deformation of thefirst optical element produced by the actuator. Other devices thatmeasure the deformation and/or the forces producing the deformations arecontemplated as well.

In another advantageous embodiment a plurality of actuators aredistributed over an area, through which projection light is allowed topropagate, of a surface of the first optical element which is oppositeto a surface which is in contact with the liquid. In this way theactuators do not get in the contact with the liquid, and thus undesiredinteractions between the liquid and the actuators are prevented. If theactuators are formed by layer actuators, they do not require anothertransparent optical member to rest on. If conventional actuators areused, a rigid optical element of a refractive type may be provided onwhich the plurality of actuators rest.

The variety of possible deformations may be further increased byproviding a further interspace formed between the second optical elementand a third optical element. Both the second optical element and thethird optical element are of a refractive type. Means, for example afurther actuator coupled to the third optical element, are provided fordeforming the third optical element. The deformation of the thirdoptical is preferably, but not necessarily, rotationally asymmetric aswell. Such a configuration may also be advantageous if the interspacesare not filled by a liquid, but by gases.

In addition to the actuators, the pressure of the liquid may be used forcausing a deformation of the first optical element. To this end a devicefor changing the pressure of the liquid, such as a pump or a deformablemembrane, may be used. This concept is also applicable if a gas is usedinstead of the liquid.

The first optical element may have a shape in its undeformed state whichis rotationally asymmetric. If the pressure of the adjacent liquid ischanged, this will cause a rotationally asymmetric deformation that maycorrect rotationally asymmetric wavefront deformations. A rotationallyasymmetric shape of the first optical element may also be useful,however, if it is (solely) deformed with the help of actuators.

The rotationally asymmetric shape may either be the result of arotationally asymmetric contour or of a thickness distribution which is,in the undeformed state of the first optical element, rotationallyasymmetric. In the latter case it may be advantageous if the firstoptical element has a first surface which is in contact to the liquidand has a rotationally asymmetric shape. A second surface opposite thefirst surface has a rotationally symmetric shape. In its undeformedstate such a first optical element has a rotationally symmetric opticaleffect, because the rotationally asymmetric surface is in contact to theliquid and thus does not (substantially) contribute to the overalloptical effect.

Also this concept of providing a rotationally asymmetric thicknessdistribution may be advantageously used with other kinds ofmanipulators, for example manipulators in which the interspace is notfilled by a liquid, but by a gas, or in which deformations are onlycaused by gas pressure changes.

In order to drive the at least one actuator, the projection exposureapparatus may comprise a controller which is connected to a sensorarrangement in order to determine the image errors. The sensorarrangement may, for example, comprise a CCD sensor which can bepositioned in an image plane of the objective or in a field planeconjugate therewith. The manipulator, the controller and the sensorarrangement may together form a closed feedback loop.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present invention may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawing in which:

FIG. 1 is a meridional section in a highly schematized representationthrough a projection exposure apparatus according to the invention, witha projection objective and a manipulator arranged therein for correctingrotationally asymmetric image errors;

FIG. 2 is an enlarged simplified meridional section along line II-IIthrough the manipulator of FIG. 1;

FIG. 3 is a view from below of the manipulator shown in FIG. 2;

FIG. 4 is a meridional section through the manipulator of FIGS. 2 and 3with its deformable optical element in a deformed state;

FIG. 5 is a meridional section through a manipulator according toanother embodiment, in which the liquid-filled interspace is formedbetween lenses having curved surfaces;

FIG. 6 is a meridional section along line VI-VI through a manipulatoraccording to another embodiment comprising actuator layers;

FIG. 7 is a view from below of the manipulator shown in FIG. 6;

FIG. 8 is a view from below of a manipulator according to anotherembodiment comprising actuator layers distributed over the entireoptical surface;

FIG. 9 is a meridional section through a manipulator according toanother embodiment wherein actuators are immersed in a liquid;

FIG. 10 is a meridional section along line X-X through a manipulatoraccording to still another embodiment comprising actuators that aredistributed over the entire optical surface, but outside an interspacefilled with liquid;

FIG. 11 is a view from below of the manipulator shown in FIG. 10;

FIG. 12 is a meridional section along line XII-XII through a manipulatoraccording to another embodiment comprising two deformable opticalelements;

FIG. 13 is a view from below of the manipulator shown in FIG. 12;

FIG. 14 shows the manipulator of FIGS. 12 and 13 with deformed opticalsurfaces;

FIG. 15 is a meridional section through a manipulator according toanother embodiment in which deformations are also caused by pressurevariations;

FIG. 16 is a meridional section through a manipulator according to stillanother embodiment comprising a deformable member having a rotationallyasymmetric shape;

FIG. 17 is a meridional section through a manipulator according to stillanother embodiment comprising a flexible membrane separating two fluidsfrom each other;

FIG. 18 is a meridional section through a manipulator according to astill further embodiment comprising a member which is deformable both bypressure variations and actuators.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows, in a highly schematized meridian section, amicrolithographic projection exposure apparatus 10 in a projection mode.The projection exposure apparatus 10 comprises an illuminating system 12for generating projection light 13. The illumination system 12 containsa light source 14, illumination optics 16 and a field stop 18. Theillumination optics 16 make it possible to set different illuminationangle distributions.

The projection exposure apparatus 10 further comprises a projectionobjective 20, which contains an aperture stop AS and a plurality ofoptical elements. For the sake of clarity, only a few optical elementsare schematically illustrated in FIG. 1 and denoted by L1 to L6. Theprojection objective 20 projects a reduced image of a mask 24, which isarranged in an object plane 22 of the projection objective 20, onto aphotosensitive layer 26 which is arranged in an image plane 28 of theprojection objective 20. The photosensitive layer 26 may be formed by aphotoresist applied on a wafer 30.

The optical elements L1, L2 and L3 are provided with actuator systemsA1, A2 and A3 (indicated only schematically) which can change theoptical effect of the optical elements L1, L2 or L3. In this embodimentthe optical element L1 is a biconvex lens that can be moved within theXY plane with a high accuracy by the actuator system A1. The actuatorsystem A2 makes it possible to change the position of the opticalelement L2, which is a biconvex lens, too, along the directionperpendicular thereto, i.e. along the Z axis. The optical element L3 isa plane-parallel plate that forms, together with the actuator system A3,a manipulator M1 which will be explained in more detail below withreference to FIGS. 2 to 4.

The actuator systems A1 to A3 are connected via signal lines 341 to 343to a controller 36 that individually controls the actuator systems A1 toA3. To this end, the controller 36 comprises a computer 38 thatdetermines which control instructions should be communicated to theactuator systems A1 to A3 in order to improve the imaging properties ofthe projection objective 20.

The projection objective 20 represented here by way of example isassumed to be telecentric on the image side. This means that the exitpupil lies at infinity. The term exit pupil refers to the image-sideimage of the aperture stop AS. In FIG. 1, rays R1, R2 represented bydots indicate how points in a pupil plane 32, in which the aperture stopAS is arranged, are imaged at infinity by the subsequent opticalelements L4 to L6.

Even with careful mounting and adjustment of the projection objective20, it will generally have image errors which degrade the imaging of themask 24 onto the photosensitive layer 26. There may be various causes ofthe image errors.

On the one hand, there are image errors which result from the design ofthe projection objective 20, i.e. in particular from the specificationof the dimensions, materials and spacings of the optical elementscontained in the projection objective 20. One example of this is theintrinsic birefringence of calcium fluoride (CaF₂), which becomesincreasingly noticeable at wavelengths shorter than 200 nm. The effectof birefringence is generally that the polarization state of theprojection light changes in an undesirable way when it passes throughthe birefringent material.

On the other hand, there are image errors which are attributable toproduction or material defects. Generally this kind of image errors canonly be corrected once the projection objective has finally beenmounted. Examples of production defects include so-called form defects,i.e. deviations from a real surface from the shape specified by thedesigner. Material defects do not, at least generally, affect thecondition of refractive or reflective surfaces, but usually lead toinhomogeneous refractive index profiles or locally varying birefringenceproperties inside the optical elements. Material defects may be due toimperfections of the materials from which the optical elements are made.Sometimes, however, these defects are not present at the beginning, butoccur after many hours or months of operation. Often this kind ofmaterial defects is caused by the high-energy projection light thatirreversibly compacts the lens material.

There are also image errors which do not occur until during theprojection operation but are reversible in nature, and therefore recedeafter the end of the projection operation. The most important cause ofsuch image errors is the heat input by projection light. This heatabsorbed by the lens or mirror material often leads to an inhomogeneoustemperature distribution and therefore also to a deformation of theoptical elements. The deformation is usually not rotationally symmetric,but often has an m-fold symmetry where m=2, 3, 4, . . . . As statedabove, an m-fold symmetry means that the optical element has itsoriginal shape again after a rotation through 360°/m. For projectionexposure apparatus in which the mask is illuminated by a slit-shapedillumination field, m is often 2.

Form defects and deformations of the optical elements lead to wavefrontdeformations, i.e. deviations from the ideal waveform in the exit pupil.In the following it is described how such wavefront deformations can beat least partially corrected with the manipulator M1.

First, the projection exposure apparatus is converted from a projectionmode into a measurement mode. A shearing interferometer 40, which allowsvery rapid analysis of the imaging properties of the projectionobjective 20, is integrated into the projection exposure apparatus 10.The shearing interferometer 40 uses the illumination system 12 and aspecial test mask. When the projection exposure apparatus is convertedinto the measurement mode, the test mask is introduced into the objectplane 22 of the projection objective with the aid of a firstdisplacement device 45 which is usually referred to as a reticle stage.The test mask thus replaces the mask 24 that shall finally be projected.With a second displacing device 42 (wafer stage) for moving the wafer 30parallel to the image plane 28, the wafer 30 is replaced by adiffraction grating. Other parts of the shearing interferometer 40, i.e.a photosensitive sensor 44 which may for example be a CCD chip, arearranged inside the displacing device 42. The function of the shearinginterferometer 40 is known as such in the art, see, for example, US2002/0001088 A1, and will therefore not be described in further detail.For selected field points, the shearing interferometer 40 makes itpossible to determine the wavefront in the exit pupil. The greater theimage errors in the projection objective 20 are, the more the wavefrontmeasured for a field point will deviate from the ideal waveform in theexit pupil.

As a matter of course, any other of the plurality of known devices andmethods for determining image errors may be used instead or additionallyto the shearing interferometer 40.

In the embodiment shown, it is assumed that a wavefront deformation dueto an astigmatic image error can be described in the exit pupil (for aparticular field point) by a Zernike polynomial Z₅. Such a wavefrontdeformation with twofold symmetry cannot generally be corrected by theoptical elements L1 and L2 which can be moved with the aid of theactuators A1 and A2.

The computer 38 in the controller 36 of the projection exposureapparatus 10 now compares the measured actual wavefront profile with astored target wavefront profile in a comparator 46 and, on the basis ofany deviations found, calculates a suitable bending of the opticalelement L3 so that the wavefront deformations are reduced for the fieldpoint in question. The calculated bending leads to wavefrontdeformations which are complementary with the measured wavefrontdeformations and therefore compensate for them. Preferably thesecalculations are repeated for a plurality of field points, and a shapeof the optical element L3 is determined that reduces, on the averageusing a certain averaging function (arithmetic, quadratic or withweighting coefficients, for example), the measured wavefrontdeformations for all contemplated field points. The actuator A3 is thencontrolled such that a compensating deformation with an opposite effectis imposed on the wavefronts in the projection objective 20.

The structure of the manipulators M1 will be explained in more detailbelow with reference to FIGS. 2 and 3, which show the manipulator M1 onan enlarged scale in a meridian section along the line II-II and a viewfrom below, respectively.

The manipulator M1 comprises a first plane-parallel plate 50 and asecond plane-parallel plate 52, arranged in parallel and spaced apart bya small distance. The two plates 50, 52 each consist of a material whichis transparent at the wavelength λ of the projection light. For awavelength λ=193 nm, an example of a suitable material for the plates50, 52 is synthetic quartz glass which has a refractive index n_(SiO2)of approximately 1.56. Materials such as LiF, NaF, CaF₂ or MgF₂ are alsosuitable. For a wavelength λ=157 nm, materials such as CaF₂ or BaF₂should be used because synthetic quartz glass is not sufficientlytransparent at these short wavelengths.

The two plates 50, 52 are framed so that an interspace 54 remainingbetween them is sealed in a fluid-tight fashion. In the sectionalrepresentation of FIG. 2, a first frame 56 for the first plate 50 and asecond frame 58 for the second plate 52 are represented in a simplifiedway as annular frames which are kept apart by an intermediate ring 60.The intermediate ring 60 contains seals (not shown in detail) whichensure fluid-tight closure of the interspace 54.

The interspace 54 is completely filled with a liquid 62. In theembodiment shown, the interspace 54 has a relatively large height forthe sake of clarity. However, it is often more favorable for the twoplates 50, 52 to be separated from each other merely by a liquid film.The thickness of the film, measured along the direction of an opticalaxis OA of the projection objective, may be as small as about 2 μm.

The liquid 62 may, for example, be highly pure deionized water which hasa refractive index n_(H2O) of approximately 1.44 at a temperature of 22°C. If the second plate 52 consists of quartz glass, then the refractiveindex ratio V=n_(SiO2)/n_(H2O) at the upper optical surface 64 of thesecond plate 52 in FIG. 1 is less than 1.01, which corresponds to arelative refractive index difference of 1%. Owing to this refractiveindex ratio V of close to 1 at the upper surface 64 of the second plate52, the upper surface 64 has only a minimal optical effect. The effectof the upper optical surface 64 can be reduced even further by bringingthe refractive indices of the liquid 62 on the one hand, and thematerial of the second plate 52 on the other hand, even closer to eachother. If water is used as the liquid 62 and, for example, LiF which hasa refractive index n_(LiF) of approximately 1.44 at a wavelength of 193nm is used as the material for the second plate 52, then the relativerefractive index ratio V=n_(LiF)/n_(H2O) is less than 0.1%.

Other substances may be envisioned as liquid 62 as well. All liquidsthat have been proposed as immersion liquids for microlithographicimmersion objectives are principally suitable, for example. Since theliquid 62 does not get into contact with the photoresist or any otherphotosensitive layer 26, there are no restrictions as far compatibilitywith the photoresist is concerned.

The interspace 54 is in communication with a reservoir 68 through achannel 66 formed in the intermediate ring 60. The reservoir 68 isarranged so that the liquid level lies above the interspace 54 when theprojection objective 20, with the manipulator M1 installed in it, is inthe operating state. In this way, the pressure of the liquid 62 in theinterspace 54 is equal to the sum of the pressure of the surrounding gasvolume and the hydrostatic pressure which the liquid 62 generates in thereservoir 68.

The interspace 54 may also be connected to a drain channel (not shown)through which the liquid 62 supplied from the reservoir is drained. Bothchannels may be part of a closed circulation system that containspurifying means for purifying the liquid 62 and also temperature controlmeans for keeping the temperature of the liquid 62 at a desired targettemperature. The temperature of the liquid 62 may be controlled suchthat the refractive index ratio V is as close to 1 as possible.Additionally or alternatively, the temperature of the liquid 62 may beused as an additional parameter for varying the optical effect producedby the manipulator M1.

On the other side from the intermediate ring 60, the second frame 58 forthe second plate 52 is adjoined by a holding ring 70 for actuators 711to 718, which together form the actuator system AS3. The distribution ofthe actuators 711 to 718 over the circumference of the second plate 52can be seen more clearly in the view from below in FIG. 3. The actuators711 to 718 are represented in a simplified fashion as threaded pins,which are adhesively bonded or connected in another way to the secondplate 52. Via geared transmissions (not shown in detail) in the holdingring 70, each individual threaded pin of the actuators 711 to 718 can bemoved independently of one another in the manner of a micrometer drivealong the longitudinal axis of the threaded pins with a very highaccuracy, as indicated by double arrows in FIG. 2. The actuators 711 to718 can thereby exert bending moments on the second plate 52, which leadto its deformation. The shape of the deformation is determined by theway in which the actuators 711 to 718 are arranged along thecircumference of the second plate 52, and by the direction and size ofthe forces exerted on the second plate 52.

The representation of the actuators 711 to 718 as threaded pins in FIGS.2 and 3 is merely exemplary. Other types of actuators, for example piezoelements, are generally used in order to exert particularly fine-tunedforces. Other examples of actuators will be described further below withreference to FIGS. 6 to 10.

In the state represented in FIGS. 2 and 3, it is assumed that theactuators 711 to 718 are not exerting any bending moments on the secondplate 52 so that it has its original configuration, which isplane-parallel in this embodiment. If the actuators 711 and 715, whichlie opposite each other in the meridian plane shown in FIG. 2, are nowactuated by screwing the threaded pins downward out of the holding ring70, then the second plate 52 will bend as shown in FIG. 4. The actuators712 and 714, and 716 and 718, are readjusted accordingly so that theyare connected force-free to the second plate 52. In this state as shownin FIG. 4, the second plate 52 is deformed in a saddle fashion withtwofold symmetry. Such a deformation is suitable for correcting awavefront deformation that can be described by the Zernike polynomialZ₅, which may be caused by a slit-shaped illumination field on the mask24.

For the sake of clarity, the deformation of the second plate 52 due tothe actuators 711 to 718 is represented greatly exaggerated in FIG. 4.In fact, bending by about 500 or even a less than 50 nanometers may besufficient to achieve the desired wavefront deformation. This highsensitivity is mainly due to the fact that, owing to the refractiveindex ratio lying close to 1, the upper optical surface 64 of the secondplate 52 does not compensate for the optical effect which originatesfrom the lower optical surface 72.

The volume of the interspace 54 generally changes in the event of adeformation of the second plate 52. With the deformation shown in FIG.4, for example, a slight volume increase of this volume takes place. Inorder to prevent compressive forces exerted on the first plate 50through the fluid 62 during the deformation of the second plate 52, theliquid 62 is allowed to flow in from the reservoir 68 through thechannel 66.

The resultant lowering of the liquid level in the reservoir 68 isrepresented exaggeratedly in FIG. 4; in fact, the volume changes whichoccur are so small that the liquid level in the reservoir 68 changesonly insignificantly. The hydrostatic pressure therefore remains almostconstant in the interspace 54. This ensures that the second plate 52 canbe deformed entirely independently of the first plate 50 with the aid ofthe actuators 711 to 718.

If it is found during the analysis with the aid of the shearinginterferometer 40 that a different deformation of the second plate 52 isnecessary in order to correct the image errors, then the controller 36may suitably drive the actuators 711 to 718 another way. A deformationwith threefold symmetry, for example, can be generated by simultaneouslyactuating the actuators 712, 715 and 718, which respectively make anangle of 120° between one another. The actuators 711, 714 and 716angularly offset by 60° thereto may execute a correspondingcountermovement. It is furthermore possible to superpose a plurality ofdeformations, so that even fairly complex wavefront deformations canbecome corrected. To this end, the excursions of the individualactuators 711 to 718 are simply added up.

FIG. 5 shows another embodiment for a manipulator denoted in itsentirety by M2 in a meridional section similar to FIG. 2. For like partsuse is made of the same reference numerals as in FIGS. 2 to 4, and forparts corresponding to one another use is made of reference numeralsaugmented by 200. In the manipulator M2 the upper plane-parallel plate50 of the embodiment shown in FIGS. 2 to 4 is replaced by a thickmeniscus lens 250, and the lower plane-parallel plate 52 is replaced bya thin meniscus lens 252. The liquid film in the interspace 254 istherefore curved. The manipulator M2 consequently has, in its entirety,the effect of a meniscus lens. The thick meniscus lens 250 mayalternatively or additionally be deformed. To this end, FIG. 5 indicatesactuators 2711, 2712 which can generate bending moments in the thickmeniscus lens 250. The manipulator M2 therefore allows more versatilecorrection of wavefront deformations.

FIGS. 6 and 7 show another embodiment of a manipulator M3 in ameridional section along line VI-VI and in a view from below,respectively. For like parts the same reference numerals as in FIGS. 2to 4 are used, and for parts corresponding to one another use is made ofreference numerals augmented by 300. The manipulator M3 is equallysuitable for being positioned within the projection objective 20 closeto the pupil plane 32 or at other positions along the optical axis OA.

The manipulator M3 differs from the manipulators M1 and M2 describedabove mainly in that it comprises a different kind of actuators that donot need to be supported on a fixed and rigid counter member, such asthe holding ring 70 of the previous embodiments. More specifically, themanipulator M3 comprises eight actuator layers 3711 to 3718 that solelyrest on the lower optical surface 372 of the second plane-parallel plate352. As can best be seen in FIG. 7, the actuator layers 3711 to 3718have, in the embodiment shown, the shape of ring segments that arearranged close to the circumference of the second plate 352 such thatthey form a ring interrupted by slit-like gaps. However, the segments ofactuator layers 3711 to 3718 may also be configured such that they abutto adjacent elements which results in a quasi-continuous actuator ring.

Each actuator layer 3711 to 3718 is formed in this embodiment by a piezoelectric element. The crystals of the piezo electric elements may befixed to the lower surface 372 of the second plate 352 using additionalconnector layers, glues, bonding, fusion bonding, solding or opticalcontacting. Since the actuator layers 3711 to 3718 are arrangedimmediately adjacent the second frame 58, electrical conductors forsupplying an electrical voltage to the piezo electric elements can beattached or received within the second frame 58.

The crystals of the piezo electric elements are aligned such that, uponapplication of an electrical voltage, tensile or compressive forces areproduced by the actuator layers 3711 to 3718 along a tangentialdirection indicated by double arrows in FIG. 7. The directions of theforces therefore extend in a plane which is perpendicular to the opticalaxis OA. This is different to the actuators 711 to 718 of the previousembodiments where forces parallel to the optical axis OA are produced.

Since the actuator layers 3711 to 3718 are arranged only on one side ofthe second plate 352, there will be an asymmetric force distributionwithin the second plate 352 which causes its bending. This even holdstrue if the forces produced by the actuator layers 3711 to 3718 aresubstantially rotationally symmetric. The more actuator layers beingindividually controllable are arranged around the circumference of thesecond plate 352, the more different deformations may be produced by theactuator layers. It is to be understood that the actuator layers 3711 to3718 do not have to have the same geometry, but may be adapted tospecific desired deformations that are required for correcting certainimage errors.

Apart from not requiring the holding ring 70 or a similar countermember, the actuator layers 3711 to 3718 are flatter and thereforerequire less room within the projection objective 20.

In an alternative embodiment, the actuator layers 3711 to 3718 are notformed by piezo electric elements, but by layers having a coefficient ofthermal expansion which is different from the second plate 352. If suchlayers are heated, for example by applying an electrical voltage or byillumination with a laser beam, the change of length of such layersresults again in compressive or tensile forces on the second plate 352in a plane perpendicular to the optical axis OA. Therefore the secondplate 352 will deform as a result of the different coefficients ofthermal expansion.

If the manipulator M3 is part of a closed feedback control loop, thesecond plate 352 may be deformed as long as it is necessary to reducethe image errors below a tolerable threshold. However, the correctionprocess may be accelerated if information is obtained how the secondplate 352 has been actually deformed by the actuator layers 3711 to3718. To this end sensors 380 may be arranged on the upper opticalsurface 364 of the second plate 352 around its circumference in asimilar way as the actuator layers 3711 to 3718 are arranged on thelower optical surface 372. The sensors 380 make it possible to measurethe deformation produced by the actuator layers 3711 to 3718. Themeasured thickness data produced by the sensors 380 are preferablysupplied to the controller 36 so that the actuator layers 3711 to 3718may be adjusted to reduce any differences between the measureddeformations and the calculated target deformation required forcorrecting the image errors.

The sensors 380 may also be formed by piezo electric elements. Theseelements produce a voltage if they are subjected to compressive ortensile forces. Other kinds of sensors 380 are contemplated as well, forexample resistant strain gauges that change their electrical resistancedepending on the strain across the gauges.

As a matter of course, the positions of the actuator layers 3711 to 3718and the sensors 380 may be reversed, i.e. the actuator layers 3711 to3718 may be applied to the upper optical surface 364, and the sensorsmay be formed on the lower optical surface 372. The choice on which sidethese elements are applied may depend on the compatibility of theelements with the liquid 62.

In a still further alternative embodiment, actuator layers are appliedon both the upper optical surface 364 and the lower optical surface 372.Such a configuration is expedient if larger deformations of the secondplate 352 have to be available. For correcting stronger wavefrontdeformations, as they are caused by asymmetric lens heating, forexample, the required deformations of the second plate 352 may be aslarge as about 500 nm.

FIG. 8 is a view from below of a manipulator M4 according to a furtherembodiment of the invention. For like parts the same reference numeralsas in FIGS. 2 to 4 are used, and for parts corresponding to one anotheruse is made of reference numerals augmented by 400. The manipulator M4is equally suitable for being positioned within the projection objective20 close to the pupil plane 32 or also at other positions along theoptical axis OA. The manipulator M4 differs from the manipulator M3shown in FIGS. 6 and 7 in that a large plurality of actuator layers 471are distributed over the entire lower surface 472 of the second plate452.

If the actuator layers 471 are not or only partially transparent at thewavelength of the projection light, the manipulator M4 has to bearranged in or in close proximity to a pupil plane of the projectionobjective 20. The projection light absorbed by the actuator layers 471will then cause a reduction of the light intensity in the image plane 28of the projection lens 20, but does not produce image errors, as wouldbe the case with an arrangement further away from a pupil plane. Inorder to keep the light losses small, the overall area of the actuatorlayers 471 should not exceed 30%, preferably 10%, of the area of theplate 52 through which projection light may propagate.

Light losses may be (at least substantially) avoided if the actuatorlayers 471 and the necessary electrical wiring (not shown in FIG. 8) aretransparent at the wavelength of the projection light. In this case itmay even be envisaged to arrange the manipulator M4 outside a pupilplane of the projection projective 20. Transparent actuator layers 471may be formed by crystalline quartz elements that display a piezoelectric effect. Electrical wiring for the piezo electric elements maybe made of indium tin oxide (ITO) which is transparent at wavelengthsbelow 200 nm. If the electrical wiring is formed by very thin conductivestripes, also non-transparent conductive materials such as aluminum maybe used without substantially increasing light losses due to absorptionby the electrical wiring.

Since the actuator layers 471 are distributed over the entire loweroptical surface 472 of the second plate 452, it is possible to producealmost any arbitrary deformation of the second plate 452. The moreactuator layers 471 are provided, the larger is the variety ofdeformations that may be produced. The manipulator M4 therefore makes itpossible to correct also very complicated wavefront deformations thatcan be adequately described only with higher order Zernike polynomials.

In the embodiment shown in FIG. 8 the actuator layers 471 commonly forma Cartesian grid so that compressive and/or tensile forces may beapplied to the second plate 52 in two orthogonal directions, as isindicated in FIG. 8 by double arrows 482, 484. As a matter of course,other configurations of the actuator layers 471 may be chosen dependingon the expected image errors and the deformations of the second plate 52required for correcting those errors. For example, the actuator layers471 may be formed as stripes that radially extend from the optical axisOA to the rim of the second plate 452. This configuration may have anm-fold symmetry adapted to the symmetry of the wavefront deformationsthat shall be corrected by the deformation of the second plate 452.Another alternative for a suitable configuration is to arrange stripesof actuator layers so that a honeycombed pattern is obtained.

It should be noted that, although the embodiments described hereinaboveall contain a liquid adjacent to a deformable optical surface, theprinciple of applying thin actuator layers to a deformable opticalsurface may be seen in a broader sense. Therefore thin actuator layersmay be applied also on those refractive or reflective optical surfacesthat are not in contact with a liquid.

FIG. 9 shows a manipulator M5 according to still another embodiment in ameridional section similar to FIG. 2. For like parts the same referencenumerals as in FIGS. 2 to 4 are used, and for parts corresponding to oneanother use is made of reference numerals augmented by 500. Themanipulator M5 is equally suitable for being positioned within theprojection objective 20 close to the pupil plane 32 or also at otherpositions along the optical axis OA.

The manipulator M5 differs from the manipulator M1 shown in FIGS. 2 to 4mainly in that actuators 571 are not arranged on the lower opticalsurface 572 of the second plate 552, but on its upper optical surface564. The actuators 571 are therefore arranged within the interspace 554and completely immersed in the liquid 62. Furthermore, the actuators 571are not arranged around the circumference of the second plate 52, butdistributed over its entire upper optical surface 564. Similar to themanipulator M4 shown in FIG. 8, this implies that the manipulator M5should be, due to absorption caused by the actuators 571, arranged in orin close proximity to a pupil plane of the projection objective 20.Apart from that the actuators. 571 should be either transparent at thewavelength of the projection light, or the overall area covered by theactuators 571 should be kept small in order to reduce light losses andto avoid problems caused by heat resulting from light absorption.

The actuators 571 may be arranged in a grid-like fashion on the upperoptical surface 564 of the second plate 552 so that almost any arbitrarydeformation of the second plate 552 is achievable. The electrical wiring(not shown) required for individually controlling the actuators 571 ispreferably applied to the lower surface of the rigid first plate 550which does not significantly deform if the actuators 571 are operated.This reduces the risk of damages of the electrical wiring caused bydeformations.

The actuators 571 may be realized as piezo electric elements, forexample made of crystalline quartz. The piezo electric elements areoriented such that their length along the optical axis OA varies inresponse to the applied voltage. Alternatively, elements may be usedthat change their length along this direction depending on theirtemperature. The temperature may be varied by an electrical currentflowing through these elements, or by an external laser, for example.

FIGS. 10 and 11 show a manipulator M6 according to still anotherembodiment in a meridional section across line X-X and in a view frombelow, respectively. For like parts the same reference numerals as inFIGS. 2 to 4 are used, and for parts corresponding to one another use ismade of reference numerals augmented by 600. The manipulator M6 isequally suitable for being positioned within the projection objective 20close to the pupil plane 32 or also at other positions along the opticalaxis OA.

The manipulator M6 mainly differs from the manipulator M5 shown in FIG.9 in that the actuators 671 are not distributed within the interspace654 which is filled with the liquid 62. Instead, the actuators 671 aredistributed over the lower optical surface 672 of the second plate 652.In this embodiment the actuators 671 are assumed to change their lengthalong the optical axis OA and therefore have to rest on a rigid countermember. This counter member is realized as a third transparentplane-parallel plate 688 received in the holding ring 70 adjacent thesecond plate 652. The third plate 688 has a thickness such that it isnot, or at least not significantly, deformable by the actuators 671. Ifthe operation of the actuators 671 results in a change of their lengthalong the optical axis OA, this will therefore only result in adeformation of the second plate 652, but not of the third plate 688. Iflayer actuators are used as in the embodiments shown in FIGS. 6 to 8,the third plate 688 may be dispensed with.

The arrangement of the actuators 671 outside the liquid 62 isadvantageous for various reasons. Firstly, the actuators 671 are notimmersed in a liquid, but surrounded by a gas, for example an inert gassuch as nitrogen. Therefore no care has to be taken in the selection ofthe liquid 62 and the actuators 671 whether the liquid 62 may becontaminated by the actuators 671, and/or whether the liquid 62 maydamage the actuators 671. Apart from that, any problems resulting fromthe contact of the electrical wiring (not shown) with a liquid arecompletely avoided. Furthermore, the fastening of the actuators 671 tothe adjacent plates is greatly facilitated because many fasteningtechnologies are susceptible to damages caused by surrounding liquids.For that reason the actuators 671 may be fastened to the second plate652 and the third plate 688 by optical bonding, which is generallydifficult or even impossible in the case of the manipulator M5 shown inFIG. 9. Another advantage is that the thin deformable second plate 652is now sandwiched between two thicker and more rigid plates that protectthe deformable second plate 652 from damages during assembly andmaintenance work.

The following aspects may be considered when designing the manipulatorM6:

The deformable second plate 652 should be as thin as possible so as toreduce the forces, and therefore the number and size of the actuators671, which are required for causing a deformation thereof. On the otherhand the first plate 650 and the third plate 688 should be as rigid aspossible. The number of actuators 671 should be as small as possible inorder to minimize the overall actuator area and thus the light losses inthe pupil plane. The actuators 671 should furthermore have a low powerconsumption and should not display a significant drift behavior in spiteof their small size. Apart from piezo electric elements that have beenmentioned above, Lorenz actuators or thermo-elastic actuators oftenfulfill these criteria. The actuators 671 may be produced on the thirdplate 688 using CVD/PVD methods, sputtering or etch processes. Similarconsiderations also apply to the other embodiments shown in FIGS. 6 to9.

In the foregoing it has been assumed that the first plate 650, thesecond plate 652 and the third plate 688 are received in annular framesin a sealed fashion. However, other methods for mounting these platesmay be used instead, for example isostatic mounting or mounts withresilient legs, as are known in the art as such. A sealed enclosure ofthe interspace 654 filled with a liquid 62 may be achieved by bellows,similar to what is shown below in FIG. 16.

The exemplary arrangement of the actuators 671 as shown in FIG. 11 maybe varied in various ways. Instead of arranging the actuators 671 as aregular grid, configurations may be considered which are specificallyadapted to the expected wavefront deformations requiring correction.

FIGS. 12 and 13 show a manipulator M7 according to another embodiment ina meridional cross section along line XII-XII and a view from below. Forlike parts the same reference numerals as in FIGS. 2 to 4 are used, andfor parts corresponding to one another use is made of reference numeralsaugmented by 700. The manipulator M7 is equally suitable for beingpositioned within the projection objective 20 close to the pupil plane32 or also at other positions along the optical axis OA.

The manipulator M7 mainly differs from the manipulator M1 shown in FIGS.2 to 4 in that the first thick plate 50 is replaced by a thin firstplate 750 which is deformable upon operation of a plurality of actuatorsfirst 7711′ to 7718′. The first actuators 7711′ to 7718′ are received ina second holding ring 70′ which is connected to the first frame 56. Themanipulator M7 therefore has two optical surfaces that can beindividually deformed. Since these deformations are caused independentlyby first actuators 7711′ to 7718′ and second actuators 7711 to 7718 andnot (only) by pressure changes of the liquid 62, a wide variety ofdeformations can be produced.

FIG. 14 shows the manipulator M7 in a configuration in which the firstand second deformable plates 750, 752, respectively, are individuallydeformed. Here it is assumed that the second actuators 7711 to 7718 arecontrolled in such a way that the second plate 752 has, in its deformedconfiguration, a two-fold rotational symmetry, as it typically occurswith illumination fields having an elongated slit-like shape. The firstplate 750 is assumed to have, in its deformed configuration, a shapewith a three-fold rotational symmetry. In this respect it is noted thatthe second actuators 7111 to 7118 for the second plate 752 are notarranged around the circumference of the second plate 752 in the sameway as the first actuators 7111′ to 7118′ acting on the first plate 750.Thus the first plate 750 may be deformed in a manner that cannot beachieved with the second plate 752, and vice versa.

FIG. 15 shows a manipulator M8 according to a still further embodimentin a meridional section. For like parts the same reference numerals asin FIGS. 2 to 4 are used, and for parts corresponding to one another useis made of reference numerals augmented by 800. The manipulator M8 isequally suitable for being positioned within the projection objective 20close to the pupil plane 32 or at other positions along the optical axisOA.

The manipulator M8 differs from the manipulator M7 shown in FIGS. 12 to14 mainly in that the interspace 54 is divided into a first interspace854′ and a second interspace 854 by a rigid plane-parallel plate 86. Thedeformable members adjacent the first and second interspaces 854′, 584are formed in this embodiment by a pellicle 850 and a thin meniscus lens852. In the meridional section of FIG. 15 first actuators 8711′ to 8714′can be seen that are received in a first holding ring 70′ for deformingthe pellicle 850. Second actuators 8711 to 8715 can be seen that arereceived in a second holding ring 70 for deforming the meniscus lens852.

The rigid plate 86 reliably ensures that deformations of the meniscuslens 852 do not cause, via pressure fluctuations in the liquid,deformations of the pellicle 850, and vice versa.

The provision of two completely decoupled interspaces 854, 854′ filledwith liquids 62, 62′ makes it furthermore possible to deform thepellicle 850 and the meniscus lens 852 independently from each other byvarying the pressure of the liquids 62, 62′. To this end the manipulatorM8 comprises two pumps 860, 860′ that are connected to reservoirs 68,68′ containing the liquids 62 and 62′, respectively, and to channels 66,66′ leading into the interspaces 854, 854′. By appropriately controllingthe pumps 860, 860′ it is possible to independently vary the pressure ofthe liquids 62, 62′ in the interspaces 854, 854′.

The pressure controlled by the pumps 860, 860′ therefore provides anadditional parameter that may be used, in addition to the first andsecond actuators 8711′ to 8714′ and 8711 to 8715, for giving thepellicle 850 and the meniscus lens 852 the shape that is required forcorrecting image errors. The deformation caused by the pressurevariation superimposes with the deformation caused by the first andsecond actuators 8711′ to 8714′ and 8711 to 8715, respectively. Theprovision of the separated interspaces 854, 854′ and the pumps 860, 860′therefore makes it possible to still further increase the range ofdeformations that may be achieved with the manipulator M8.

As a matter of course, instead of the pumps 860, 860′ various othermeans may be provided for changing the pressure of the liquids 62, 62′.For example, hydraulic cylinders or deformable membranes may be used.

The deformations that can be achieved with pressure fluctuations dependmainly on the material properties and the shape of the deformablemember, and on the way it is mounted to non-deformable members of theprojection objective 20.

Important design parameters for the shape of the deformable member areits outer contour and its thickness distribution. For example, if aplane-parallel plate has a rectangular contour, a pressure increase willresult in a deformation of the plate having a two-fold symmetry.

The deformable members of the manipulator M8, namely the pellicle 850and the meniscus lens 852, are assumed to have circular contours.Nevertheless both members 850, 852 will respond differently to pressurevariations, because both members 850, 852 have different thicknessdistributions. Since both the pellicle 850 and the meniscus lens 852 arecompletely rotationally symmetric, pressure variations within the liquid62, 62′ will necessarily cause deformations which are equallyrotationally symmetric.

If a deformable optical element has a thickness distribution which isrotationally asymmetric, then a pressure variation will also cause arotationally asymmetric deformation even if the outer contour of theelement is circular.

This concept is realized in the manipulator M9 which is shown in FIG. 16in a meridional section. For like parts the same reference numerals asin FIGS. 2 to 4 are used, and for parts corresponding to one another useis made of reference numerals augmented by 900. The manipulator M9 isequally suitable for being positioned within the projection objective 20close to the pupil plane 32 or also at other positions along the opticalaxis OA.

The manipulator M9 differs from the manipulator M8 shown in FIG. 15mainly in that the meniscus lens 852 is replaced by a lens 952 that hasa rotationally symmetric contour, but—even in its undeformed state, i.e.in the absence of any forces other than the force of gravity—arotationally asymmetric thickness distribution.

When the lens 952 is deformed by using actuators or, in this particularembodiment, solely by changing the pressure of the liquid 62 with thehelp of the pump 960, the deformation will be rotationally asymmetricdue to the rotationally asymmetric shape of the undeformed lens 952.This rotationally asymmetric shape of the undeformed lens 952 is, inthis particular embodiment, designed such that it corrects comaaberration after being deformed.

Preferably the lens 952 has a rotationally symmetric optical effect inits undeformed state. This may be achieved by providing the lens 952with an upper surface 964 which is rotationally asymmetric, whereas thelower surface 972 of the lens 952 is rotationally symmetric, for exampleplane and perpendicular to the optical axis OA, as is the case here. Ifthe liquid 62 filling the second interspace 954 has (almost) the sameindex of refraction as the lens 952, the upper rotationally asymmetricsurface 964 has no or at least no substantial optical effect. Only thelower surface 972 adjacent to a gas (or alternatively to a liquid havinga significantly distinct index of refraction) contributes to the overalloptical effect of the lens 952. In this embodiment this is the effect ofa plane-parallel plate. In embodiments with a convexly or concavelycurved lower surface 972, the lens 952 has, in its undeformed state, arotationally symmetric refractive power like a commonplace lens.

In the embodiment shown the lens 952 is mounted with the help of two ormore mounting legs 990. For sealing the second interspace 954 againstthe outside, a metal bellow 992 is sealingly connected to the lens 952and a holding ring 994. The metal bellows 992 allow for small movementsof the lens 952 resulting from pressure variations produced by the pump960.

A first interspace 954′ is formed between a rigid plane-parallel plate986 and a thin deformable plane-parallel plate 950. In this embodimentthe plate 950 can only be deformed by actuators of which four actuators9711 to 9714 can be seen in the meridional section of FIG. 16.

If the plate 950 is replaced by another lens that is obtained byrotating the lens 952 by 90° around the optical axis OA, comaaberrations of any arbitrary direction can be corrected by changing thepressure of the adjacent liquids or with the help of actuators.

FIG. 17 shows a manipulator M10 according to still another embodiment ina meridional section. For like parts the same reference numerals as inFIGS. 2 to 4 are used, and for parts corresponding to one another use ismade of reference numerals augmented by 1000. The manipulator M10 isequally suitable for being positioned within the projection objective 20close to the pupil plane 32 or also at other positions along the opticalaxis OA.

The main difference between the manipulator M10 on the one hand and themanipulators according to the previous embodiments is that thedeformable member, which is realized in this embodiment by a thinmembrane 1052, does not separate a liquid and a gaseous medium, but twoliquid mediums having different refractive indices. Again, there is onlyone refractive surface that can be deformed using actuators from whichactuators 10711 to 10715 can be seen in the meridional section of FIG.17. However, since the refractive index ratio of two liquids isgenerally quite closer to 1 than the ratio obtained with a liquid and agas, deformations of the membrane 1052 will have a much weaker opticaleffect than deformations of the deformable members that have beendescribed previously. The manipulator M10 is therefore particularlysuitable for carefully correcting small image errors with high accuracy.

In the embodiment shown a first interspace 1054′ is formed between afirst thick and rigid biconvex lens 1050 and the membrane 1052. Thefirst interspace 1054′ is filled with a first liquid 62′. A secondinterspace 1054 is formed between the membrane 1052 and a second rigidbiconvex lens 1053 and is filled with a second liquid 62. Since thesensibility of the manipulator M10 depends on the refractive index ratiobetween the first liquid 62′ and the second liquid 62, it is easilypossible—even after the manipulator M10 has been mounted in theprojection objective 20—to change this sensitivity by replacing one orboth liquids with a liquid having a different index of refraction.Alternatively, the index of refraction, and therefore the sensitivity ofthe manipulator M10, may be varied by changing the temperature of one orboth liquids 62, 62′. This exploits the fact that the refractive indexof liquids usually displaces a strong temperature dependency.

In contrast to prior art solutions where the deformable member isdeformed solely as a result of pressure variations within the liquids,the manipulator M10 makes it possible to produce a variety of completelydifferent deformations with the help of the actuators 10711 to 10715.For example, a deformation having a 3-fold symmetry may be producedfirst, and after some ours of operation an additional deformation havinga 2-fold symmetry may be superimposed under appropriate arrangement andcontrol of the actuators 10711 to 10715. The manipulator M10 istherefore suitable not only for correcting field curvature aberration,but also field independent image errors that can only be described withhigher order Zernike polynomials.

As a matter of course, the provision of actuators does not exclude theadditional provision of pressure changing means, as has been describedabove with reference to the manipulator M8 shown in FIG. 15.

If the membrane 1052 is sufficiently thin, it may have a refractiveindex that is distinct from both refractive indices of the adjacentliquids 62, 62′. Also in this embodiment the use of other deformableoptical members, for example thin plates or meniscus lenses, iscontemplated as well.

As matter of course, also in the previously described embodiments it isenvisaged to circulate intermittently or continuously the liquids withintheir respective interspaces.

FIG. 18 shows a manipulator M11 according to a still further embodimentin a meridional section. For like parts the same reference numerals asin FIGS. 2 to 4 are used, and for parts corresponding to one another useis made of reference numerals augmented by 1100. The manipulator M11 isequally suitable for being positioned within the projection objective 20close to the pupil plane 32 or also at other positions along the opticalaxis OA.

The manipulator M11 differs from the manipulator M8 shown in FIG. 15 inthat it has only one deformable optical member, namely the thin meniscuslens 1152. The meniscus lens 1152 has an upper surface 1164 which has a(weak) 2-fold rotational symmetry (not shown in FIG. 18), similar to thelens 952 of the manipulator M9 shown in FIG. 16. Actuators 11711 to11715 are arranged around the circumference of the lens 112 such that adeformation having a 3-fold symmetry may be achieved.

By independently controlling the pressure of the liquid 62 using thepump 1160 (similar pressure variation means) on the one hand and theactuators 11711 to 11715 on the other hand, deformations with 2-fold and3-fold symmetries may be successively or simultaneously be produced.Since it often suffices to be able to individually produce twodeformations having different symmetries, an additional deformablemember, for example the membrane 850 of the manipulator M8 shown in FIG.15, may be dispensed with.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. The applicant seeks, therefore, tocover all such changes and modifications as fall within the spirit andscope of the invention, as defined by the appended claims, andequivalents thereof.

1. An apparatus, comprising: a first optical element that is refractive;a second element; a liquid between the first optical element and thesecond element; and an actuator coupled to the first optical elementsuch that operation of the actuator causes a rotationally asymmetricdeformation of the first optical element substantially without causing adeformation of the second element, wherein the apparatus is amicrolithographic projection exposure apparatus.
 2. An apparatusaccording to claim 1, wherein the first optical element is aplane-parallel plate.
 3. An apparatus according to claim 1, wherein thefirst optical element is a meniscus lens.
 4. An apparatus according toclaim 1, wherein the first optical element is a membrane.
 5. Anapparatus according to claim 1, wherein: there is an interspace betweenthe first optical element and the second element and, the interspacehas, in a direction along an optical axis of a projection objective ofthe apparatus, a maximum thickness of less than 1 mm.
 6. An apparatusaccording to claim 5, wherein the maximum thickness is less than 2 μm.7. An apparatus according to claim 1, wherein the deformation of thefirst optical element has an m-fold symmetry or a superposition of aplurality of m-fold symmetries with m being an even, positive integer.8. An apparatus according to claim 1, comprising at least two actuatorsthat are configured to exert bending moments on the first opticalelement.
 9. An apparatus according to claim 8, wherein the at least twoactuators are arranged along a circumference of the first opticalelement.
 10. An apparatus according to claim 8, wherein the actuatorsare configured such that at least two deformations of the first opticalelement can be generated having a different m-fold symmetry.
 11. Anapparatus according to claim 1, wherein the second element isreflective.
 12. An apparatus according to claim 1, wherein the secondelement is refractive.
 13. An apparatus according to claim 12, whereinthe second element is a meniscus lens or a plane-parallel plate.
 14. Anapparatus according to claim 1, comprising a second actuator configuredto act exclusively on the second element to generate a deformation ofthe second element.
 15. An apparatus according to claim 14, wherein thedeformation of the second element is rotationally asymmetric.
 16. Anapparatus according to claim 14, wherein the deformation of the secondelement is rotationally symmetric.
 17. An apparatus according to claim1, comprising a pressure equalizer configured to maintain a constantpressure of the liquid in an immersion space.
 18. An apparatus accordingto claim 1, wherein the first optical element is arranged in or in closeproximity of a pupil plane.
 19. An apparatus according to claim 1,wherein the actuator is fastened to a front or a rear surface of thefirst optical element, and wherein the actuator is configured to produceon the first optical element compressive and/or tensile forces alongdirections that are at least substantially tangential to the surface.20. An apparatus according to claim 19, wherein the actuator is formedby a layer which is applied to the surface, the layer having a variabledimension along at least one direction.
 21. An apparatus according toclaim 20, wherein the layer comprises a piezo electric material.
 22. Anapparatus according to claim 20, wherein the layer comprises a materialhaving a coefficient of thermal expansion that is different from thecoefficient of thermal expansion of the first optical element.
 23. Anapparatus according to claim 22, comprising a device configured tocontrollably change the temperature of the layer.
 24. An apparatusaccording of claim 23, wherein the device is configured to directradiation onto the layer.
 25. An apparatus according of claim 23,wherein the device is configured to apply a voltage to the layer.
 26. Anapparatus according to claim 20, wherein a plurality of layers have theshape of ring segments, the ring segments forming a circular ring whichis centered with respect to an optical axis of a projection objective ofthe apparatus.
 27. An apparatus according to claim 26, wherein the ringsegments are spaced apart by gaps.
 28. An apparatus according claim 19,wherein a plurality of actuators are distributed over the surface withinan area through which projection light propagates.
 29. An apparatusaccording to claim 28, wherein the plurality of actuators are at leastsubstantially transparent at the wavelengths of the projection light.30. An apparatus according to claim 28, wherein the plurality ofactuators are opaque at the wavelengths of the projection light, andwherein the manipulator is arranged in or in close proximity to a pupilplane of a projection objective of the apparatus.
 31. An apparatusaccording to claim 1, comprising a sensor that is fastened to a front orrear surface of the first optical element, wherein the sensor isconfigured to measure a deformation of the first optical elementproduced by the actuator.
 32. An apparatus according to claim 31,wherein the sensor is configured to produce an output signal whichdepends on the amount of compressive and/or tensile forces produced inthe sensor by a deformation of the first optical element.
 33. Anapparatus according to claim 1, wherein a plurality of actuators aredistributed over an area, through which projection light is allowed topropagate, of a surface of the first optical element which is oppositeto a surface which is in contact with the liquid.
 34. An apparatusaccording to claim 33, comprising a further optical element that isrefractive on which the plurality of actuators rest.
 35. An apparatusaccording to claim 1, further comprising: a third optical element; andmeans for deforming the third optical element, wherein there is aninterspace between the second element and a third optical element, boththe second element and the third optical element are refractive.
 36. Anapparatus according to claim 1, comprising a device configured to changethe pressure of the liquid.
 37. An apparatus according to claim 1,wherein the first optical element has a shape in its undeformed statewhich is rotationally asymmetric.
 38. An apparatus according to claim37, wherein the first optical element has a thickness distribution inits undeformed state which is rotationally asymmetric.
 39. An apparatusaccording to claim 38, wherein the first optical element has a firstsurface, which is in contact to the liquid and has a rotationallyasymmetric shape, and a second surface opposite the first surface,wherein the second surface has a rotationally symmetric shape.
 40. Anapparatus according to claim 1, further comprising: a controllerconfigured to drive the actuator; and a sensor arrangement connected tothe controller, the sensor arrangement being configured to determine theimage errors.
 41. The apparatus of claim 40, wherein the sensorarrangement comprises a CCD sensor, which is positionable in an imageplane of a projection objective of the apparatus or in a field planeconjugate therewith.
 42. The apparatus of claim 41, wherein theactuator, the controller and the sensor arrangement form a closedfeedback loop.
 43. A method, comprising: providing an apparatusaccording to claim 1, the apparatus comprising an illumination systemand a projection objective; arranging a mask in an object plane of theprojection objective of the apparatus; using the illumination system toilluminate the mask with light; and after illuminating the mask withlight, using the projection objective to project the mask onto aphotosensitive layer which is arranged in an image plane of theprojection objective.
 44. An apparatus, comprising: a first opticalelement that is refractive, the first optical element having a thicknessdistribution in its undeformed state which is rotationally asymmetric, asecond element, a fluid between the first optical element and the secondelement; and means for deforming the first optical element, wherein theapparatus is a microlithographic projection exposure apparatus.
 45. Amethod, comprising: providing an apparatus according to claim 44, theapparatus comprising an illumination system and a projection objective;arranging a mask in an object plane of the projection objective of theapparatus; using the illumination system to illuminate the mask withlight; and after illuminating the mask with light, using the projectionobjective to project the mask onto a photosensitive layer which isarranged in an image plane of the projection objective.
 46. Anapparatus, comprising: a first optical element that is refractive; asecond element; a liquid between the first optical element and thesecond element; and an actuator coupled to the first optical elementsuch that operation of the actuator causes a rotationally asymmetricdeformation of the first optical element substantially without causing adeformation of the second element, wherein a ratio between therefractive index of the first optical element and the refractive indexof the liquid is between 0.9 and 1.1, and the apparatus is amicrolithographic projection exposure apparatus.
 47. An apparatusaccording to claim 46, wherein the ratio between the refractive index ofthe first optical element and the refractive index of the liquid isbetween 0.99 and 1.01.